ipv6: Fix Makefile offload objects

The following commit breaks IPv6 TCP transmission for me:
	Commit 75fe83c3
	Author: Vlad Yasevich <vyasevic@redhat.com>
	Date:   Fri Nov 16 09:41:21 2012 +0000
	ipv6: Preserve ipv6 functionality needed by NET

This patch fixes the typo "ipv6_offload" which should be
"ipv6-offload".

I don't know why not including the offload modules should
break TCP. Disabling all offload options on the NIC didn't
help. Outgoing pulseaudio traffic kept stalling.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>